https://www.vice.com/en_us/article/n7nmaz/post-office-cops-used-social-media-surveillance-program-illegally

The United States Postal Inspection Service, the law enforcement arm of the post office, was “not legally authorized” to conduct blanket keyword searches of social media for terms such as “protest,” “attack,” and “destroy,” an inspector general investigation found, because it is only supposed to investigate cases with some connection to the post office or the mail. 

The USPSIS is the oldest police agency in the country, called the United States Postal Inspection Service, or USPIS. Its job is to police the mail and crimes related to the mail such as mail fraud. USPIS dates back to a time when the postal service was the largest and most significant branch of the federal government and the USPIS was its only national policing power.